1. Field of the Invention
The present invention relates to a jet propeller provided with a step portion on a bottom surface of a stem of a watercraft in order to reduce, during planing of the watercraft, a wet surface area of the watercraft and a resistance applied to a hull of the watercraft.
2. Description of Background Art
A jet propulsion watercraft has been known, for example, from Japanese Patent Laid-Open No. 6-312684 entitled “STERN PART STRUCTURE OF JET PROPULSION WATERCRAFT.” The jet propulsion watercraft will be explained in detail with reference to FIG. 8 of the present invention. FIG. 8 of the present invention corresponds to FIGS. 5(a) and 5(b) of the above document. It should be noted that the reference numerals in FIG. 8 do not correspond to FIGS. 5(a) and 5(b) of the above document.
FIG. 8 is a cross-sectional view illustrating a jet propulsion watercraft according to the background art. A jet propulsion watercraft 100 is provided with a jet propeller 101 at a stem thereof. There is further provided a steering nozzle 102 at a rear of the jet propeller 101. When the jet propeller 101 is driven, a stream of water is expelled through an outlet port 102a of the steering nozzle 102, thus allowing the watercraft to plane on the water from the propulsion force of the expelled water.
A step H1 is formed on a bottom surface of the jet propulsion watercraft 100 by attaching a ride plate 103 to a level higher than a bottom surface 104a of a hull bottom 104 in order to reduce, during planing of the watercraft, a wet surface area of the watercraft 100 and a propulsion resistance applied to the hull of the watercraft 100. Forming the step H1 on the bottom surface makes a stream of water flow rearward from a side of the bottom surface 104a along the ride plate 103, thus reducing pressure at the rear of the step H1.
To prevent this decompression from occurring in the rear of the step H1, a clearance 105 is provided at the rear of the step H1, through which air inside the hull is supplied rearward the step H1. Forming the clearance 105 in the rear of the step H1 in this manner prevents the decompression in the rear of the step H1 from occurring. This accomplishes the function of reducing, during planing of the jet propulsion watercraft 100, the wet surface area of the watercraft 100 and the propulsion resistance applied to the watercraft 100.
In order to form the step H1 of a predetermined height in the bottom surface 104a of the hull bottom 104 and the clearance (namely a vent hole) 105 of a predetermined width in the rear of the step H1; however, it becomes necessary to enhance the accuracy of mounting the ride plate 103 to the hull bottom 104. This makes the mounting of the ride plate 103 to the hull bottom 104 labor-consuming, which hampers cost reduction.